The present invention relates to well logging, and in particular to improvements in a borehole logging tool referred to as a borehole televiewer, or BHTV. Tools of this type are described, for example, in U.S. Pat. Nos. 3,369,626 (Zemanek, Jr. issued Feb. 20, 1968), 3,478,839 (Zemanek, Jr. issued Nov. 18, 1969), and 4,463,378 (Rambow, issued July 31, 1984).
In general, borehole televiewer logging tools operate acoustically by periodically pulsing a rotating acoustic transducer to emit a sequence of acoustical pulses directionally into the borehole toward the borehole wall, and analyzing the echos which are reflected back to the tool. The amplitude of the reflected signal may then be displayed on a cathode ray tube, the display sometimes being photographed for future reference. Typically, the display represents a map of the borehole wall split along the north direction and laid out flat. Alternatively, a polar display may be produced, in which case the radius of the circular trace is determined by the time-of-flight of the acoustic pulse, thus presenting a cross-sectional profile of the borehole. Another display, similar to the amplitude display, is modulated by the time-of-flight signal rather than the amplitude signal. The latter can be converted into a pseudo-three-dimensional image by adding a slight bias to the vertical sweep according to the magnitude of the time-of-flight signal. BHTV tools typically include means for monitoring the tool orientation within the borehole, such as a fluxgate magnetometer rotating in unison with the transducer. A good technical description of a borehole televiewer suitable for use in geothermal environments may be found in "Development of a Geothermal Acoustic Borehole Televiewer", by Fred B. Heard and Tom J. Bauman, Sandia Report SAND83-0681, August 1983.
One of the principal and extremely valuable benefits furnished by the BHTV logging tool is the pseudo "visual" image of the borehole wall which it furnishes. Subtleties in the formation, bedding, bedding planes, dip, and so forth, can be observed and studied in a manner completely unavailable elsewhere. Especially in the oil industry, convention conventional optical viewing devices do not suffice, in part due to the typically extremely hostile environment, but primarily because the fluid medium in the borehole is normally opaque to optical energy.
As shown in the above-noted publications, borehole televiewers scan radially with a single transducer, thus essentially looking at a small ring encircling the transducer in the transverse plane thereof. As the borehole televiewer is then moved vertically through the borehole, the path or trail of this ring, as it moves along the borehole wall, in turn describes the wall. This description is then accumulated to generate the displays discussed above.
There has long been a need, particularly in the drilling industry, to be able to look ahead as well as sideways. For example, in drilling an oil well, the well may extend to 10,000 or 20,000 or more feet beneath the surface. Not uncommonly, articles (junk) such as tools, drill string, bits, hammers, and so forth, may be lost in the borehole. Sometimes they simply fall in from the surface. More commonly, a piece of equipment will break or become stuck in the borehole. Thus, the junk may not always be at the very bottom. Before drilling can proceed, an effort must obviously be made to remove the junk, referred to within the industry as a "fish", from the borehole. This procedure is called "fishing", and is a very sophisticated specialty in the drilling arts.
Before one can go fishing in a borehole, however, it is helpful to know where the fish is and what sort of upward profile it presents, so that the appropriate fishing tool can be selected and properly positioned. Heretofore, this type of determination has not been quite so sophisticated. Typically, it involves lowering a lead or tar block into the borehole and dropping it forcefully against the fish to make an impression in the block. The impression is then analyzed, and the attempts to remove the fish proceed accordingly.
A need therefore remains for a substantially improved method and apparatus for "looking" ahead in a visually opaque borehole environment where conventional optical imaging cannot effectively be performed. A need also remains for such a tool which can thus provide, for example, a far superior means for locating and defining lost articles in such a borehole.